Many methods are currently known for transferring print onto paper, including xerography and thermal printing. In thermal printing, a heat-sensitive paper is moved across a thermal head which transfers the image to the paper by applying localized pulses of heat, at up to 400.degree. C., in small spots to the surface of the paper. The localized hot spots activate a heat-sensitive chemical on the paper, which turns dark thus producing an image, as the paper moves across the thermal head.
Both thick film thermal heads and thin film thermal heads (thermal print heads) are known in the art, and are used for different applications.
Thick film thermal print heads provide high speed printing on thermally-sensitive paper for high speed graphics and bar code printing applications such as lottery and race track ticket printers, airline ticket printers and bar code label printers for many applications. In most of these applications, the paper to be printed is coarse and abrasive. In addition, these thermal printers are often used in situations where the environment is not well-controlled, e.g. in warehouses at race tracks, etc. In these situations, the thermal print head becomes exposed to degrading environmental conditions such as dust, high humidity and acidic vapors (from acid rain) and chemical vapors.
A typical thermal print head for these applications has a size of approximately 1 inch wide.times.4 inches long.times.1/8 inch thick, made of a ceramic substrate, such as aluminum oxide. Prior to deposition of the resistor strip, a projected glaze strip, made of glass or a glass-ceramic material may be applied to the substrate. A resistor strip comprised of a plurality of closely spaced heating elements ("dots") of resistor material (made of ruthenium oxide, tantalum oxide, titanium oxide, titanium silicide, nichrome, or other resistive material) is deposited over the substrate, and on top of the projected glaze strip, if present. The individual heating elements of the resistor strip are connected on two sides to conductor lines, which are typically made of metals such as gold or silver. For protection, the resistor strip may be encapsulated by a layer of glass or glass-ceramic glaze, having a thickness up to about 25 micrometers. Alternatively, the resistor strip may be protected by a hard coating layer of vacuum deposited ceramic material. An electric current (typically pulsed) applied via the conductor lines to the resistor dots produces resistive heating of the resistive element to a temperature in the typical range of approximately 350.degree. C. to 400.degree. C. or greater. When a heat pulse from the resistor dot comes in contact with thermally sensitive paper, the dot image is transferred onto the paper. By appropriate application of electrical pulses to the heating elements, and moving the paper across the print head, the bar code label or ticket information is printed onto the paper.
Thin film thermal print heads all have similar construction to thick film thermal print heads, except that the layers of materials used to build up the thermal print head are thinner, and normally deposited by thin film vacuum deposition technology. Thin film thermal print heads are most often used in applications where the environmental conditions are less severe, and the paper to be printed is less abrasive, e.g. in facsimile machines. A common, simple thin film thermal print head construction might entail an aluminum oxide ceramic substrate, a resistor material of nichrome which is less than 1 micrometer thick, and a protective layer of silicon nitride, which is less than 2 micrometers thick.
The susceptibility of each of the prior art thermal heads to failure after extended operation is well known. Several mechanisms contribute to premature failure of the thermal head, including removal of, or damage to the protective coating by abrasive wear, corrosion, and thermal degradation. Abrasive wear is believed to occur due to rubbing of the print head surface by hard particles such as titanium oxide particles in the paper, or unwanted debris such as sand, or other silicate or oxide materials which are present in the environment. The low hardness and high friction surfaces of prior art print heads, which are coated with protective layers such as glass, silicon oxynitride and silicon nitride, make them susceptible to abrasive damage by these particles. In addition, the thermal head may be damaged by corrosion by chemicals such as water, salts, acids and other chemicals in the paper and the environment, if the protective coating is not resistant to these materials. Finally, the thermal cycling to which the resistor material is subjected can lead to thermal degradation over time. This situation is made worse by the use of protective coatings which have poor thermal conductivity, such as glass or amorphous silicon nitride. In the search for improved wear resistance, manufacturers have attempted to increase the thickness of protective coatings such as glass or silicon nitride. Because of the poor thermal conductivity of these layers, increased electrical power must be applied to the resistor elements to make them hotter, in order achieve the same temperature at the surface of the print head to cause the color change in the thermally-sensitive paper. This increased temperature of the resistor element shortens its lifetime.
There are many configurations of thermal heads known in the prior art, all of which exhibit the aforementioned limitations.
For example, Ogawa et al., U.S. Pat. No. 4,708,915, describe a thermal head for thermal recording having a protective coating composed of tantalum silicon oxynitride. An undercoat may be formed between this protective coating and the heat-generating resistors and electrodes.
Shibata, U.S. Pat. No. 4,768,038, discloses a thermal printing head having a plurality of electrodes disposed on an insulating substrate, in an upper layer and a lower layer. The electrodes are connected to a heat generating layer between the electrodes, and are isolated by a layer of plasma-deposited silicon nitride or silicon oxide.
Sugiyama, U.S. Pat. Nos. 5,021,806 and 5,095,318, describes a thermal print head comprising a substrate; an electrically insulating layer coated over the substrate; a heating means coated over the insulating layer, for providing heat for printing a dot of a picture; a protective coating layer applied over the heating means; and a dot area control means. The protective coating layer may be an oxidation resistant material.
Nakayama, et al., U.S. Pat. No. 5,557,313, disclose a sputter-deposited wear-resistant protective film for a thermal head consisting of a metal oxide, metal nitride, and mixtures thereof, such as silicon oxynitride, wherein the coating has an inert gas concentration of 2 to 10 atomic percent.
Diamond-like carbon (DLC) coatings, which can be composed of pure carbon, or carbon and hydrogen, are well known in the prior art. These DLC materials are known to exhibit excellent mechanical properties such as high hardness of about 10 to about 80 GPa, low coefficient of friction of approximately 0.2 or less, excellent resistance to abrasion, and resistance to corrosion by water, acids, bases, and solvents. Therefore, it would be expected that DLC coatings would perform well as protective coatings on thermal print heads. However, it was found that standard DLC coatings deposited by direct ion beam deposition from methane gas were rapidly degraded and worn away during thermal printing because of the high temperatures, i.e. approximately 400.degree. C. or greater, to which the coatings were exposed during the thermal printing process.
From the above discussion it is clear that an improved protective coating for thermal print heads is needed that exhibits improved wear resistance and excellent thermal stability without sacrificing printing performance and resolution of the thermal head.